The present invention relates to a maximum-likelihood sequence estimator (MLSE) with a variable number of states for use in mobile communication or the like.
1. Description of the Related Art
In conventional radio communication, intersymbol interference due to a delayed multipath wave causes degradation of characteristics. In particular, in a digital automobile telephone system using a TDMA (time-division multiplex access) system, suppression of the intersymbol interference has been a large subject and it is necessary to adopt an equalizing technique such as a decision-feedback equalizer, a maximum-likelihood sequence estimator (MLSE) and the like. Especially, the MLSE is called a Viterbi equalizer and its equalizing ability is high. Hence, the MLSE is widely used for terminals of the European GSM and the North American IS-54.
FIG. 1 shows a conventional maximum-likelihood sequence estimator for use in a burst transmission. In FIG. 1, a received signal having a predetermined burst length is stored into a memory 100. The received signal concerning a training signal position within the burst is input from the memory 100 to a channel response estimator 101. The channel response estimator 101 calculates channel responses {hi}.sub.i=1,K from the received signal while referring to an input training signal. At this time, the number K of the channel responses is previously determined according to the maximum delay amount of a multipath wave in the worst communication environment. The channel response estimator 101 outputs the estimated channel responses {hi}.sub.i=1, K to a Viterbi equalizer 203.sub.K-1, with a fixed number M.sup.(K-1) of states. The Viterbi equalizer 203.sub.K-1 executes a maximum-likelihood sequence estimation to output a decision signal. In this case, the number M.sup.(K-1) of states is constant.
In FIG. 2, there is shown another conventional adaptive maximum-likelihood sequence estimator. In FIG. 2, a received signal is input to a channel response estimator 101. The channel response estimator 101 estimates channel responses {hi}.sub.i=1, K from the received signal while referring to either a training signal when the training is supplied or a decision signal when information transmission is carried out. The channel response estimator 101 sends the estimated transmisson line responses {hi}.sub.i=1,K to a Viterbi equalizer 203.sub.K-1 with a fixed number M.sup.(K-1) of states. The Viterbi equalizer 203.sub.K-1 carries out a maximum-likelihood sequence estimation to output a decision signal. In this case, the number M.sup.(K-1) of states is constant.
Usually, complexity of an MLSE is much and thus its reduction becomes a large subject. The MLSE is described in detail in Document 1: "Maximum-Likelihood Sequence Estimation of Digital Sequences in the Presence of Intersymbol Interference" by G. D. Forney, Jr., IEEE Trans. on Inform. Theory, Vol. IT-18, No. 3, pp. 363-378, May 1972, and Document 2: "Adaptive Maximum-Likelihood Receiver for Carrier-Modulated Data-Transmission Systems" by G. Ungerboeck, IEEE Trans. on Commun., Vol. COM-22, No. 5, pp. 624-636, May 1974.
In the MLSE, the complexity is determined by the number of states of the state transition trellis of the Viterbi algorithm used in the inside. Conventionally, the number of states of the MLSE is determined depending on the maximum delay amount of the multipath wave in the worst communication environment and thus large complexity is always required, resulting in a large load of the signal processing by the MLSE. Accordingly, a decision-feedback MLSE has been proposed, wherein the number of states of the MLSE is reduced in advance from the number of states for the worst environment and the information removed by the reduction is supplemented with the information of the survived paths, as disclosed in, for example, Document 3: "Delayed Decision-Feedback Sequence Estimation" by A. Duel-Hallen and C. Heegard, IEEE Trans. on Commun., Vol. 37, No. 5, pp. 428-436, May 1989. In this system, although the complexity is reduced by the reduction of the number of states, the number of states is determined in advance regardless of the states of channels. Hence, in the worst environment (in the case of non-minimum phases) that the power of a delayed wave having a large delay time becomes relatively larger than that of the desired wave by fading or the like, it is inevitable that with the reduction of the number of states, the characteristics are degraded.
In order to reduce a consumption power of receivers, particularly, at portable mobile terminals, it has been demanded to develop a reducing means of a processing load of an MLSE without degrading characteristics.